ARCHITECTURE OF AN INTERVAL OF COMPLEX LARGE-SCALE SOFT-SEDIMENT DEFORMATION (SSD) IN THE NAVAJO SANDSTONE NEAR KANAB, UTAH

Intervals of complex soft-sediment deformation (SSD) tens of metres thick affect large-scale cross-bedded eolianites in the Navajo Sandstone. Their mode of formation, paleoenvironmental and paleotectonic significance are poorly understood. Existing models relate to differential subsurface liquefaction controlled by surface paleodunes and paleo-interdunes.

 

We present a detailed map of an entire interval of SSD exposed in the walls of Kanab Canyon, S Utah, mapped using direct field observation and analysis of drone imagery. The interval is about 600 m in lateral extent (N-S, parallel to paleo-wind direction) and at least 30 m thick. Lower, upper and lateral transitions into undeformed cross-stratification are exposed, allowing reconstruction of the pre-deformation cross-stratification. The deformed zone is analysed by identifying architectural elements, each of which is interpreted in terms of a deformation mechanism and driving force.

 

Deformation is most intense near the centre, becoming less intense towards both lateral margins, base and top. There is some internal organization to the distribution of deformation elements. The down-wind part is dominated by large-scale shear zones that record spreading in the down-wind direction, suggesting the influence of surface morphology. A central core is dominated by vertical movements, represented by large-scale antiforms and synforms, chaotically deformed and massive zones. The massive zones likely represent fluidized sediment and deformation may have been driven by density contrasts between fluidized and denser liquefied sediment. At the margins, complex deformation passes laterally and vertically over short distances into undeformed stratification, but localized deformation features persist many metres beyond the main deformation interval, indicating that deformation died out in the subsurface rather than being truncated by deflationary erosion.

 

These elements of interpretation are assessed against existing models for SSD in the Navajo subsurface. We consider the relationship between the deformed interval and surface paleomorphology, the depth below the surface at which deformation occurred, the timing relative to sedimentation, and the possible trigger for deformation.